1. Field of the Invention
The present invention relates to a display apparatus for displaying an image produced by a beam of electrically charged particles with negligible image distortion.
2. Description of the Prior Art
The teaching of the present invention can be applied to apparatus provided with a plurality of deflection circuitries for the beams of electrically charged particles. However, for the better understanding of the invention, it will be described herein only exemplarily with reference to an electron microscope of the scanning type.
As is well known in the art, the electron microscope of the scanning type incorporates therein two deflection circuitries for electron beams. One of the deflection circuits is disposed in the body of the microscope and serves to scan a specimen with the accelerated primary electron beam bi-dimensionally. Then, the secondary electrons emitted by the specimen illuminated with the primary electrons are detected by a detector, the output signal of which is fed to a grid electrode of a cathode ray tube (hereinafter simply called CRT). The other one of the deflection circuit is disposed in the CRT and serves to deflect the electron beam modulated in dependence on the applied signal to thereby produce a magnified image of the specimen on the fluorescent screen of the CRT.
In general, the scanning speed of the scanning type electron microscope is slow as compared with the case of a television receiver. For example, the generation of a field of image by the scanning with the electron beam in the CRT for the electron microscope requires in most cases about 0.2 sec. With such slow scanning speed, flickers will occur in the produced image and provide difficulty for the observation. To evade such flickers, CRT having an image retaining property is employed at present. In case of the photographing, several tens seconds are required for the exposure to obtain a single image.
The above problem of the occurrence of flickers may be solved merely by increasing the scanning speed of the electron beam. However, when the scanning speed is increased to the degree at which the flickers will disappear, there will arise another problem that distortion will be produced in the image and make it impossible to obtain a magnified image of the specimen with a reliability required for the scanning type electron microscope. The cause of such distortion of the image can be explained by the fact that there is difference in the inductance value of the deflection coils disposed in the microscope and the CRT. In more detail, the scanning signals applied to the both deflection coil systems are synchronized with each other. When the deflection current is supplied to these deflection coils which have different inductance values, the wave forms of voltages appearing across these deflection coils will have non-similarity to each other, which gives rise to distortion in the magnified image displayed on the CRT. Such distortion becomes more remarkable, as the scanning speed is increased or the difference between the inductance values of the both coils becomes greater.
The difference in the inductance values of the deflection coils for the microscope and the CRT is ascridable to the inherently differed constructions and deflection ranges of the electron beams and thus is inevitable. For a numerical example, the inductance value of the deflection coil for the electron microscope is in the order of 0.1 mH, while that of the deflection coil for CRT is about 1 mH.